Over the years I've converted to the belief that what is important in language and grammar is that the communication is not unintentionally ambiguous, not that it satisfies any formal criterion. Whether you say your mum or my mum, no one is going to be confused by what you mean. So use whichever feels right to you.
A: I just spent $5 on the worst cup of coffee ever.
B: If I were you, I'd want my money back.
Seems totally wrong to me to say If I were you, I'd want your money back, but formally it is completely analogous to saying If I were you, I'd apologize to your mum.
On the other hand, I agree with keshlam that it is harder to get confused when you say "your mum" because if B were A and stayed A throughout the sentence (as in the coffee dialog example), who's mother would be "your mum"? There's no "you" left in that situation, so obviously B has popped out of the hypothetical position by that point and "your mum" must still refer to A's mum.
Also, saying "your mum" feels more empathetic and keeps the focus on A's mum rather than discussing how B would treat B's mum, so I would probably say "your mum" even though I would never say "your money".
Back to the coffee dialog, "my mum" could mean either A's mum or B's mum, but in either case the message is the same: If I had said bad things to my mum, I would apologize to my mum. The idea that A should apologize to B's mum is so absurd that the statement If I were you, I'd apologize to my mum is not ambiguous in practice. No one would ever think that B is suggesting A get B's money back for the coffee, right?
So either way the message is clear. That's all that really matters.